Machines for packaging pourable food products—such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc.—are known, on which packages are formed from a continuous tube of packaging material defined by a longitudinally sealed web.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. And, in the case of aseptic packages for long-storage products, e.g. UHT milk, the packaging material comprises a layer of barrier material defined, for example, by aluminium foil, and which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package and therefore contacting the food product.
To produce aseptic packages, the web of packaging material is unwound off a reel and fed through a sterilizing unit, in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent, such as a concentrated hydrogen peroxide and water solution.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent, into which the web is fed continuously. The bath conveniently comprises two vertical parallel branches connected at the bottom to define a U-shaped path long enough to ensure the packaging material is treated for a sufficient length of time. For effective treatment in a relatively short time, and therefore to reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature, e.g. around 70° C.
The sterilizing unit also comprises a process chamber located over the bath, and in which the web of packaging material is dried; and an aseptic chamber, in which the web is folded and sealed longitudinally to form a tube, which is then filled continuously with the product for packaging.
More specifically, in the process chamber, the web is processed to remove any residual sterilizing agent, the acceptable amount of which in the packaged product is governed by strict standards (the maximum permissible amount being in the region of a few fractions of apart per million).
Such processing normally comprises mechanical removal of any drops on the material, followed by air drying.
The drops may be removed, for example, by feeding the material through a pair of wringing rollers conveniently located close to the process chamber inlet, and downstream from which the material is still covered with a film of sterilizing agent, but has no macroscopic drops.
Drying may be performed by means of air knives facing opposite faces of the material, supplied with air from the sterile environment, e.g. by means of a recirculating conduit as described in EP-A-1 050 467, and which provide for removing residual traces of sterilizing agent by evaporation.
Alternatively, complete drying may be achieved in a low drying channel, through which the process chamber communicates with the aseptic chamber.
Before leaving the aseptic chamber, the web is folded into a cylinder and sealed longitudinally to form, in known manner, a continuous, longitudinally sealed, vertical tube. In other words, the tube of packaging material forms an extension of the aseptic chamber, and is filled continuously with the pourable food product and then fed to a forming and (transverse) sealing unit for forming the individual packages, and on which the tube is gripped and sealed transversely between pairs of jaws to form aseptic pillow packs.
The pillow packs are separated by cutting the seals between the packs, and are then fed to a final folding station where they are folded mechanically into the finished shape.
Packaging machines of the above type are used widely and satisfactorily in a wide range of food industries for producing aseptic packages from web-fed packaging material. Performance of the sterilizing unit, in particular, ensures ample conformance with standards governing sterility of the packages.
A need for further improvement, however, is felt within the industry itself, particularly as regards pressure control in the sterilizing unit.
In known machines, the pressure and temperature conditions in the process and aseptic chambers are normally controlled by a closed air processing circuit, which draws air from the process chamber and feeds it back into the aseptic chamber.
To ensure sterility of the environment defined by the process and aseptic chambers, both chambers must be maintained at higher than atmospheric pressure, so that any leakage can only occur outwards, i.e. sterile air can leak from the machine, but no non-sterile air can leak from the outside environment into the machine. Moreover, to ensure one-way airflow from the aseptic chamber to the process chamber, at least a roughly 10 mmH2O pressure difference must be maintained between the two chambers.
In known machines, the pressure values in the aseptic and process chambers are substantially defined by design conditions, are ensured by appropriate calibrated leakage between the two chambers and between the process chamber and the outside, and are simply monitored, so that, if they are too low, e.g. due to in-service sealing defects, the machine, and therefore production, must be stopped for the necessary steps to be taken.